Electrostatic toner polymer particles are commonly prepared by suspension polymerization in a process commonly referred to as "limited coalescence". In this process, polymer particles having a narrow size distribution are obtained by forming a solution of a polymer in a solvent that is immiscible with water, dispersing the solution so formed in an aqueous medium containing a solid colloidal stabilizer and removing the solvent by evaporation The resultant particles are then isolated, washed and dried.
In the practice of this technique, toner particles are prepared from any type of polymer that is soluble in a solvent that is immiscible with water. Thus, the size and size distribution of the resulting particles can be predetermined and controlled by the relative quantities of the particular polymer employed, the solvent, the quantity and size of the water insoluble particulate suspension stabilizer and the size to which the solvent-polymer droplets are reduced by the agitation employed.
Suspension polymerization techniques of this type have been described in numerous patents pertaining to the preparation of electrostatographic toner particles because such techniques typically result in the formation of toner particles having a substantially uniform size and uniform size distribution. Representative suspension polymerization processes employed in toner preparation are described in U.S. Pat. Nos. 4,314,932, 4,360,611, 4,415,644, and 4,789,617.
U.S. Pat. No 4,789,617 is representative of the prior art in this field and describes a process for the preparation of electrostatographic toner particles by solution polymerization. This process involves dispersing a polymerizable monomer, a colorant and a low softening point compound in an aqueous medium heated to a temperature above the polymerization temperature to form particles of the monomer composition and then adding a water insoluble polymerization initiator to the aqueous medium to effect solution polymerization. This results in the formation of polymerization toners which are spherical in nature, of required fluidity, and evidence excellent anti-blocking characteristics and sharp particle size distribution. These toners contain large amounts of a low softening point compound having a softening point within the range of 40.degree.-130.degree. C. Examples of the low softening point compound employed are paraffins, waxes, low molecular weight polyolefins, modified waxes having an aromatic group, natural waxes, and long chain carboxylic acids having a long hydrocarbon chain including 12 or more carbon atoms. Among the waxes described are beeswax, carnauba wax and montan wax. The low softening point compounds employed are used in an amount ranging from 50- 3000 parts by weight to 100 parts by weight of polymerizable monomer. The net result of this prior art technique is the production of spherical toner particles of sharp particle size distribution which evidence excellent fluidity and anti-blocking properties.
The shape of the toner particles prepared in accordance with the foregoing prior art technique and that of the aforementioned representative patents is generally spherical, especially when the size of the particles is smaller than 10 microns. This is of particular concern to those skilled in the art since it is also known that particle size and shape have a bearing upon the electrostatic toner transfer properties. Thus, for example, the transfer efficiency of toner particles has been found to improve as the sphericity of the particles is reduced Accordingly, workers in the art have long sought to modify the shape of the evaporative limited coalescence type toners independently of pigment or binder choice in order to enhance the transfer properties of the toner.